Vacuum evaporation is well known as one of basic thin-film forming processes. In the vacuum evaporation process, an evaporation source and a substrate are placed in a vacuum chamber, and a source material is evaporated to deposit a thin film on the substrate. A variety of evaporation sources are known. One typical process is a resistance heating evaporation process of conducting electric current across a metal container or boat having a relatively high electric resistance to generate heat with which a source material is evaporated, as described in Appl. Phys. Lett., 68 (16), Apr. 15, 1996, for example. Also known is an electron beam/laser beam evaporation process of directly irradiating electron beams or laser beams to a source material for evaporating the material with the beam energy. Of these, the resistance heating evaporation process is widely used in the art because the deposition apparatus is of simple construction so that thin films of quality can be formed at a low cost.
In the resistance heating evaporation process, a metal material having a high melting point such as tungsten, tantalum or molybdenum is worked into a thin plate having a high electric resistance, from which a container or boat is made. A source material is placed in the container, which is disposed in a (vacuum) chamber. Direct current is conducted across the container to generate heat, with which the source material is evaporated to feed a source material gas. A part of the dispersing gas deposits on the substrate to form a thin film. As the source material to be evaporated, any of materials having a relatively high vapor pressure may be used although the material that is chemically reactive with the container should be avoided.
Recently, active research works have been made on organic EL devices. As a basic configuration, the organic EL device includes a hole injecting electrode, a thin film formed thereon by depositing a hole transporting material such as triphenyldiamine (TPD), a light emitting layer deposited thereon of a fluorescent material such as an aluminum quinolinol complex (Alq3), and a metal electrode or electron injecting electrode formed thereon from a metal having a low work function such as magnesium. Such organic EL devices are attractive in that they can achieve a very high luminance ranging from several 100 to several 10,000 cd/m.sup.2 with a drive voltage of approximately 10 volts.
In the prior art method of manufacturing organic EL device-applied products, functional thin films of organic materials are formed using the evaporation process. It is crucial for such commercial mass-scale manufacture to increase the productivity and to reduce the percent rejection. However, manufacturing apparatus using prior art evaporation devices are difficult to achieve mass production and to manufacture products of uniformity and hence, high quality because of a low film deposition rate and non-uniformity in thickness and composition of organic layers during the mass-scale manufacturing process. When a functional thin film such as an electron injecting electrode is deposited on the organic layer, the organic layer can be damaged or inversely, the electron injecting electrode itself be contaminated with impurities or oxidized. These lead to defectives such as non-uniform luminance, dot defects, and current leakage as well as quality variances.
The evaporation boat is easy to control the rate of evaporation since direct resistance heating is possible. The boat, however, can accommodate therein only a small amount of a source material, lacking a practical utility from the industrial aspect.
On the other hand, a cell type evaporation source can contain a larger amount of source material, but is low in thermal response because of indirect heating. As a consequence, it is difficult to control the rate of evaporation. The percent utilization of the source material becomes low when the rate of evaporation is set constant. This makes it difficult to reduce the cost of products particularly when an expensive organic material is used. Also, in the case of evaporation at a relatively low temperature from the cell type evaporation source as in the deposition of organic layers in organic EL devices, the thermal response is further exacerbated because of poor radiating efficiency.
In particular, light emitting layers of organic EL devices are often formed by doping a host material with a minor amount of fluorescent material so as to adjust to the desired luminous characteristics. Even a slight shift in the amount of host material or dopant in the mixed layer can jeopardize the luminous characteristics. For these and other reasons, prior art evaporation equipment are difficult to achieve uniformity of products or produce EL devices of high quality, especially in the mass-scale manufacture process.